1. Field of the Invention
The present invention relates to a paper machine felt for the support and dewatering of running paper webs in paper machines as well as a method of manufacture for paper machine felts. Such felts include a lower carrier layer, also known as a base, and an upper layer of fibers, also known as a bat.
2. Description of the Related Art
The function of the carrier layer is to impart tensile strength to the felt in machine direction as well as in cross machine direction. The function of the bat layer is to support the paper web. In addition, both layers must remain permeable to water while under compression, and the carrier layer should be capable of storing water when compressed. Such felts can be made of wool, plastic, plastic fiber, or a blend of such materials.
In felts currently most frequently used, the carrier layer or base includes one or several layers of plastic fabric. To the top side of the base, one or several layers of random fiber bat layers are needled. During the needling operation, a mechanical bond is formed between bat and base by sharp-edged needles which have tooth-like notches in their edges that pull fibers from the bat and plug them vertically into or through the base or carrier layer. After the needles are retracted, the fibers remain stuck in the base by friction forces.
These types of felts are suited for a wide range of applications, but in some applications bottlenecks in dewatering become apparent. In particular, the extraction of water by air purging by use of suction boxes and suction rolls becomes increasingly ineffectual with rising machine running speed because the duration of the suction pulse diminishes.
In order to compensate for this shortcoming, the bat layers of the felts are designed more open and coarser. As a consequence, the uniformity of paper support is reduced. Also, in wet presses of paper machines, "felt marking" of the paper surface by the felt surface occurs, which is detrimental to paper quality. Further, the dryness of the paper after wet pressing is reduced by increasing backflow of water from the felt to the paper on the outgoing end of the pressing zone.
Furthermore, the bat fibers plugged into the interstices of the base layer retain water similarly to paint brushes by use of their large free surface area. Between these bat fibers, dirt particles and other matter can get stuck, reducing the water storage capacity of the interstices in the base layer still further.
The net water storage capacity of the felt, defined as the maximum water content minus the remaining water content after a water removing step, is to a great deal influenced by the number and length of the fibers plugged into the interstices of the base. Such fibers determine the strength of the bond between bat and base.
The negative effect of bat fibers within the interstices or cavities of the base was recognized a long time ago, and as an attempted solution to this deficiency, a product is described by Edward Race in APR (Aligemeine Papier-Rundschau) No. 37/38, 1970, pages 1378 to 1388. This product is intended to overcome the mentioned deficiencies. In particular, see page 1386, middle column, last paragraph; page 1388, first column; and FIG. 6.
Despite a special needling technique, the storage capacity of this product is small in comparison with the thickness of the base, and the felt is required to be sufficiently coarse in order to be dewatered by air purging over suction box slots. This felt design, however, as a consequence of manufacturing problems, did not prevail. This felt design was succeeded by open felt structures with coarse bat layers attached to both sides of a stiff base layer fabric.